1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system and, in particular, to an image forming apparatus including a sheet feed roller for feeding a sheet toward the transfer position of an image.
2. Description of the Related Art
Generally, in an image forming apparatus using an electrophotographic system, an electrostatic latent image corresponding to an image signal is formed on a photosensitive material and is then developed to thereby produce a toner image, the toner image is transferred to a sheet and, after then, the toner image on the sheet is fixed.
Also, among the image forming apparatus of this type, there is known an image forming apparatus employing a feed system in which a sheet is fed to the transfer position of a photo sensitive material or an intermediate transfer material by a regi-roller provided on this side (upstream side of sheet feeding).
In particular, in the image forming apparatus employing the above-mentioned feed system, the sheet fed from the upstream side of the sheet feeding is inserted and held by and between the regi-roller and a pinch roller in pressure contact with the regi-roller and, in this state, the regi-roller is rotated integrally with a drive shaft supporting the regi-roller, thereby feeding the sheet to the transfer position of the image.
Also, in feeding the sheet to the transfer position of the image, the rotation speed of the regi-roller is controlled, or, after the sheet is stopped provisionally at or before a position where the regi-roller is provided, a timing for removing the stop condition (that is, a sheet re-feed timing) is controlled, so that the leading end of the sheet can be made to reach the transfer position of the image at a good timing.
However, in the above-mentioned conventional image forming apparatus, in feeding the sheet to the image transfer position by the regi-roller, because the regi-roller becomes eccentric, that is, because the center of rotation of the regi-roller is shifted in position from the center of rotation of the drive shaft, when the leading end of the sheet arrives at the image transfer position, the required number of rotations of the regi-roller varies each time. That is, when the required number of rotations of the regi-roller varies each time, even if the rotation speed of the regi-roller is controlled or the sheet re-feed timing is controlled in the above-mentioned manner, in fact, the timing, at which the leading end of the sheet arrives at the image transfer position, varies from its expected timing, which makes it impossible to position or register the sheet with high accuracy.
Also, the eccentricity of the regi-roller depends on the working precision of the roller itself, the working precision of the drive shaft supporting the roller, and an assembling precision with which the regi-roller is fitted with the drive shaft; but, in fact, as a matter of course, there is a limit to enhancement of such precision. Therefore, in the conventional image forming apparatus, it is impossible to avoid the variations in the arriving timing of the sheet leading end at the image transfer position caused by the eccentricity of the regi-roller.